The present invention relates to a method and an apparatus for correcting aberration occurring in recording/reproducing information by using reflected light of an irradiating converged light beam.
In accordance with development of Internet and spread of digital cameras and digital videos, more and more image data are processed by using computers, and hence, there is a demand for a storage with a large capacity. Optical disks are most suitably used for this purpose because of their large capacities, compatibility and ability of random accesses, and various examinations have been made on a digital video disk (DVD) and the like for attaining a larger capacity.
In general, it is necessary to minimize a focal spot in order to increase a capacity of an optical disk. Since a spot diameter is in proportion to xcex/NA (wherein xcex is a wavelength and NA is numerical aperture), the spot diameter can be decreased by increasing the NA. However, when the NA is larger, aberration derived from a tilt or aberration derived from an error in the thickness of a substrate is disadvantageously increased.
For example, in a DVD using an objective lens with high NA of approximately 0.6, a laminated substrate with a thickness of 0.6 mm is used in order to decrease the aberration derived from a tilt of the substrate. Also, in an optical disk of the next generation adopting a magnetic field modulation method, a single substrate with a thickness of 0.6 mm is similarly used. The thickness of the substrate is preferably further decreased in order to further increase the NA, but the tilt can be increased in a thinner single substrate because the thin substrate can be warped. In an optical disk using a substrate with a thickness of 0.1 mm or less, a reinforcing substrate is required to be adhered onto a surface other than the surface a magnetic film is adhered to. Thus, the tilt can be decreased, but a distance between the magnetic film and an optical head is increased by the thickness of the reinforcing substrate, and hence, it is difficult to adopt the magnetic field modulation method for a recording operation. Also, a substrate with a thickness of 0.1 mm or less is difficult to make compatible with a conventional recording/reproducing apparatus because the thickness is different from that of a conventional substrate.
In this manner, when the NA is large, an effect, in particular, aberration occurring in a recording/reproducing operation derived from a tilt, a difference in the thickness of respective optical disks and thickness variation in a direction along tracks, is increased. As a result, a light beam cannot be completely converged.
A tilt of an optical disk is corrected by detecting the tilt in a recording/reproducing operation of an optical disk and controlling, for example, an inclination of an objective lens in accordance with the detected tilt. In this correction, a tilt is detected by using reflected light of light irradiating the optical disk on a portion not relation with recording/reproducing information then, so that a relative tilt between a recording/reproducing laser beam and the optical disk can be corrected in accordance with the detected tilt. When the tilt is thus corrected, information can be accurately recorded/reproduced. However, since there is a distance on the optical disk between a portion for detecting the tilt and a portion for recording/reproducing information, a tilt in the portion for recording/reproducing information cannot be accurately detected. Furthermore, the correction accuracy is largely affected by an assembling error of an optical system for detecting a tilt and an optical system for recording/reproducing information.
Moreover, a tilt of an optical recording medium along a crosswise direction of tracks thereon (hereinafter referred to as a radial tilt) is comparatively easily detected. However, a tilt in a direction crossing the crosswise direction of the tracks, for example, a tilt along a tangential direction of the tracks (hereinafter referred to as a tangential tilt) is mostly constituted by an A/C component having one rotation of the disk as one cycle, and cannot be accurately corrected by the aforementioned correction method because the detecting point is different from the correcting point, Furthermore, since a response speed is low, high speed control cannot be realized. Also, since a tangential tilt is so small as compared with a radial tilt that it has been conventionally not necessary to correct the tangential tilt. However, in accordance with increase in a memory density of an optical recording medium, the correction of a tangential tilt has become more and more significant.
Japanese Patent Application Laid-Open No. 6-162541 (1994) discloses an apparatus for correcting a tangential tilt. This apparatus is a recording/reproducing apparatus for an optical disk, in which light reflected by a flat area on an optical disk is received by a photodetector so as to detect a tilt on the basis of brightness in each divided area included in the photodetector. An objective lens for converging a laser beam on the optical disk is provided with actuators on its four sides for focus control, tracking control and tilt control. These four actuators are independently driven in accordance with a detected tilt, so that a radial tilt and a tangential tilt can be corrected.
In this apparatus, a tangential tilt can be accurately corrected through high speed control. However, since a tilt is detected on the flat area of the optical disk, the tilt correction cannot be performed at the same time as a recording/reproducing operation of information.
The present invention was devised to overcome the aforementioned problems, and one object of the invention is providing a method and an apparatus for aberration correction in which information can be precisely recorded and reproduced by detecting aberrations derived from each of causes such as thickness variation of a substrate, a tilt, a focus error and a tracking error and by correcting the respective causes on the basis of the detected aberrations.
Another object of the invention is providing a method and an apparatus for aberration correction in which an aberration derived from a tilt is detected so that the tilt can be corrected without using a complicated detecting mechanism and can be suppressed even when an objective lens has high NA and a substrate has a small thickness.
Still another object of the invention is providing a method and an apparatus for correcting thickness variation of a substrate of an optical recording medium can be corrected by detecting the aberration.
Still another object of the invention is providing an apparatus for correcting aberration in which composing elements of an optical system can be simplified by using a photodetector both for focus control and tracking control through detection of aberrations.
The method of this invention of correcting aberration occurring in recording/reproducing information by using reflected light of a light beam irradiating an optical recording medium, comprises the steps of detecting aberrations derived from each of various causes on the basis of brightness distribution in interference areas formed by diffraction light of the reflected light; and correcting aberration occurring in recording/reproducing information by correcting the respective causes in accordance with the detected aberrations.
Accordingly, the reflected light of the light beam for being irradiated the opticaloptical recording medium forms, owing to diffraction light, interference areas having brightness distribution along a track crosswise direction on optical means for converging the light beam, such as an objective lens. The present inventors have found that the brightness distribution is peculiar to a cause of aberration occurring in recording/reproducing information. In consideration of the brightness distribution, namely, the diffraction pattern, being peculiar to each cause of aberration, the extents of aberrations derived from the respective causes are detected. Detection signals thus obtained are used as control signals for correcting the respective causes, and the irradiated position with the light beam and the tilt against the substrate are changed so as to make the amplitude of these control signals zero. Thus, the aberrations can be corrected.
Alternatively, the method of correcting aberration comprises the steps of detecting an aberration derived from a relative tilt between the light beam and the optical recording medium on the basis of brightness distribution in interference areas formed by diffraction light of the reflected light; and correcting aberration occurring in recording/reproducing information by correcting the tilt in accordance with the detected aberration.
Accordingly, an aberration derived from a tilt is detected on the basis of a diffraction pattern peculiar to the aberration derived from a tilt. The tilt can be corrected on the basis of a detection signal thus obtained. Since the tilt can be corrected by using a servo circuit in an optical recording/reproducing apparatus, the entire apparatus can be made compact, and the tilt can be suppressed even when the optical recording medium includes a thick substrate.
Alternatively, the method of correcting aberration comprises the steps of detecting an aberration derived from a focus error on the basis of brightness distribution in interference areas formed by diffraction light of the reflected light; and correcting aberration occurring in recording/reproducing information by correcting the focus error in accordance with the detected aberration.
Accordingly, an aberration derived from a focus error is detected on the basis of a diffraction pattern peculiar to the aberration derived from a focus error. The focus error can be corrected on the basis of a detection signal thus obtained. Since the focus control can be conducted by using the same photodetector used for tracking error control, the entire apparatus can be made compact.
Alternatively, the method of correcting aberration of this invention comprises the steps of detecting an aberration derived from thickness variation of a substrate included in the optical recording medium on the basis of brightness distribution in interference areas formed by diffraction light of the reflected light; and correcting aberration occurring in recording/reproducing information by correcting the aberration derived from thickness variation of the substrate through focus control in accordance with the detected aberration.
Accordingly, an aberration derived from thickness variation is detected on the basis of a diffraction pattern peculiar to the aberration derived from thickness variation of a substrate. The aberration derived from the substrate thickness variation can be corrected on the basis of a detection signal thus obtained, and the effect of the thickness variation can be eliminated.
In the method of correcting aberration, in the step of detecting aberration, a detection signal of a relative tilt between the optical recording medium and the light beam, a detection signal of thickness variation of a substrate included in the optical recording medium and of a focus error and a detection signal of a tracking error are obtained, and the step of correcting aberration includes, prior to correction of the tracking error, a pseudo focus control step of controlling the light beam to be focused on the optical recording medium; a step of obtaining a median of an amplitude of the detection signal of the thickness variation and of the focus error obtained in the pseudo focus control step; a step of correcting the tracking error by using the detection signal of the tracking error; a step of correcting the focus error by using the detection signal of the thickness variation and of the focus error with the median used as a reference value; and a step of correcting the tilt by using the detection signal of the tilt.
Accordingly, the aberrations derived from a tilt, substrate thickness variation, a focus error and a tracking error are corrected by detecting the aberrations resulting from these causes, and hence, the aberration occurring in recording/reproducing information can be corrected.
In the method of correcting aberration, the pseudo focus control step includes a step of changing a distance between optical means for converging the light beam and the optical recording medium; and a step of conducting focus control on the basis of a focus control signal obtained through another path while changing the distance.
Accordingly, before correcting a tracking error, rough focus control is conducted on the basis of a focus control signal detected by, for example, the Foucault method as a process prior to precise focus control.
In the method of correcting aberration, the pseudo focus control step includes a step of changing a distance between optical means for converging the light beam and the optical recording medium; and a step of conducting focus control so as to make zero a difference between a predetermined value and a level of the detection signal of the tracking error obtained while changing the distance.
Accordingly, before correcting a tracking error, rough focus control is conducted by using a detection signal corresponding to an aberration derived from a tracking error as a process prior to precise focus control. As a result, there is no need to provide the apparatus with a general focus controller, and hence, the entire apparatus can be made compact.
Alternatively, the apparatus of this invention for correcting aberration occurring in recording/reproducing information by using reflected light of a light beam irradiating an optical recording medium, comprises at least one photodetector for detecting aberrations derived from each of various causes on the basis of brightness distribution in interference areas formed by diffraction light of the reflected light; and a correcting part for correcting the respective causes on the basis of the detected aberrations.
Accordingly, the extents of the aberrations derived from respective causes are detected by the photodetector on the basis of the brightness distribution peculiar to each cause. Detection signals thus obtained are used as control signals for correcting the respective causes of the aberrations, and the irradiated position with the light beam and the tilt against a substrate are changed so as to make the amplitude of the control signals zero. Thus, aberration can be corrected.
Alternatively, the apparatus for correcting aberration of this invention comprises an objective lens for converging a light beam; an actuator for three-dimensionally moving the objective lens; and an aberration correcting part receiving the reflected light, for detecting aberrations derived from each of various causes and for correcting the respective causes, wherein the aberration correcting part includes at least one photodetector for outputting aberration detection signals corresponding to the respective causes; and at least one controller selected from a tilt controller for correcting a tilt, a thickness/focus controller for correcting thickness variation of a substrate and a focus error, and a tracking controller for correcting a tracking error.
Accordingly, aberration occurring in recording/reproducing information can be corrected by correcting at least one of a tilt, thickness variation of a substrate, a focus error and a tracking error in accordance with the detected aberration.
In the apparatus for correcting aberration, the tilt is a tilt along a first direction having an inclination against the track crosswise direction of the optical recording medium, and at least one of the photodetectors includes, on a light receiving surface thereof, a first light receiving area formed on one side along the track crosswise direction in the reflected light and a second light receiving area formed on the other side, and each of the first and second light receiving areas is divided into an inner area and an outer area.
Accordingly, since each of the first and second light receiving areas, on the light receiving surface of the photodetector, corresponding to the interference areas of the reflected light is divided into the inner area and the outer area on the basis of an interference pattern peculiar to the aberration derived from a tilt along the first direction having an inclination against the track crosswise direction, namely, a radial tilt. Therefore, a radial tilt can be detected.
In the apparatus for correcting aberration, a first aberration detection signal RTLTES corresponding to a tilt along the first direction is calculated as follows:
RTLTES=(A1+A2+k1(B3+B4))xe2x88x92k2((A3+A4)+k1(B1+B2))
Accordingly, the light receiving surface of the photodetector is divided on the basis of an interference pattern peculiar to the aberration derived from a radial tilt, and the aberration detection signal corresponding to a radial tilt is obtained by using brightness in these divided areas.
In the apparatus for correcting aberration, at least one of the photodetectors includes, on a light receiving surface thereof, a first light receiving area formed on one side along the track crosswise direction in the reflected light and a second light receiving area formed on the other side, each of the first and second light receiving areas is divided into a center area and an edge area relative to the light receiving surface, and a second aberration detection signal THES corresponding to thickness variation of the substrate and a focus error is calculated as follows:
THES=(A1+B1+k1(A3+B3))xe2x88x92k2((A2+B2)+k1(A4+B4))
Accordingly, since the light receiving surface of the photodetector is divided on the basis of an interference pattern peculiar to the aberration derived from substrate thickness variation and a focus error, the aberration detection signal corresponding to the thickness variation and a focus error can be obtained.
Still another object of the invention is providing an apparatus for correcting aberration capable of an accurate seeking operation through detection of a seeking direction of an optical head on an optical recording medium by detecting aberration.
The apparatus of this invention for correcting aberration further comprises a beam movement direction detecting circuit, which receives the second aberration detection signal corresponding to thickness variation of the substrate and a focus error and a third aberration detection signal corresponding to a tracking error, and detects a movement direction of the light beam on the optical recording medium based on whether the second aberration detection signal is at a maximum peak or a minimum peak when the third aberration detection signal is varied in a predetermined manner.
Accordingly, since the movement direction of a head against the optical recording medium can be detected in a seeking operation in recording/reproducing information in and from the optical recording medium, it is possible to avoid a mistake in counting the number of tracks. The aberration detection signal corresponding to a focus error has a reverse polarity in scanning a land to the polarity in scanning a groove. Therefore, the movement direction of the light beam can be detected by comparing this signal with the aberration detection signal corresponding to a tracking error having the same polarity.
Still another object is providing an apparatus for correcting aberration capable of detecting a tilt along a direction crossing the track crosswise direction by using a photodetector whose light receiving surface is rotated by substantially 90 degrees on the same plane.
In the apparatus for correcting aberration, the at least one photodetector receives the reflected light dividedly along a direction crossing the track crosswise direction, and the apparatus further comprises a second tilt control circuit which receives the first aberration detection signal corresponding to a tilt and the second aberration detection signal corresponding to a tracking error, detects a tilt along a direction crossing the track crosswise direction on the basis of a level of the first aberration detection signal attained when the second aberration detection signal is at a level of zero, and corrects the detected tilt.
Accordingly, since a tilt along the direction crossing the track crosswise direction can be detected, the entire tilt can be further decreased by correcting this tilt together with a tilt along the track crosswise direction. A tilt along a direction substantially parallel to the tracks can be detected by disposing the photodetector so as to receive dividing the reflected light on the light receiving surface along the direction crossing the track crosswise direction. An optical recording medium used in this case includes a groove extending along the track crosswise direction.
In the apparatus for correcting aberration, the tilt is a tilt along a second direction having an inclination against a direction crossing the track crosswise direction of the optical recording medium, and at least one of the photodetectors includes, on a light receiving surface thereof, a first light receiving area formed on one side along the track crosswise direction in the reflected light and a second light receiving area formed on the other side, each of the first and second light receiving areas is divided into a center area and an edge area relative to the light receiving surface, and each of the center and edge areas is divided into two divided areas along the direction crossing the track crosswise direction.
Accordingly, each of the first and second light receiving areas on the light receiving surface corresponding to the interference areas of the reflected light is divided into the center area and the edge area, and each of the center area and the edge area is further divided along the direction crossing the track crosswise direction. Therefore, the photodetector is divided on the basis of an interference pattern peculiar to the aberration derived from a tilt along the second direction having an inclination against the direction crossing the track crosswise direction, namely, a tangential tilt. Thus, a tangential tilt can be detected. For example, when each of the first and second light receiving areas is in a substantially elliptical shape, each light receiving area is divided by crossed division lines crossing at the center of the elliptical shape and extending along the track crosswise direction and along a direction perpendicular to the track crosswise direction. By thus dividing the photodetector, a tangential tilt can be detected.
In the apparatus for correcting aberration, the first detection signal TTLTES corresponding to a tilt along the second direction is calculated as follows:
TTLTES=(A1a+B1a+A2b+B2b+k1)A3a+B3a+A4b+B4b))xe2x88x92k2((A2a+B2a+A1b+B1b)+k1(A4a+B4a+A3b+B3b))
Accordingly, the light receiving surface of the photodetector is divided on the basis of an interference pattern peculiar to the aberration derived from a tangential tilt, and the aberration detection signal corresponding to the tangential tilt is obtained by using brightness in these divided areas.
Still another object is providing an apparatus for correcting aberration in which a tilt correction can be performed at the same time as an information recording/reproducing operation by detecting and correcting a tilt along the track crosswise direction and a tilt along a direction crossing the track crosswise direction by using reflected light of a light beam irradiating an optical recording medium in the recording/reproducing operation.
In the apparatus for correcting aberration, each divided area of the first and second light receiving areas is further divided into an inner area and an outer area in order to detect both a tilt along the first direction and a tilt along the second direction.
Accordingly, both a radial tilt and a tangential tilt are detected by using merely one photodetector.
In the apparatus for correcting aberration, the objective lens includes a first lens and a second lens disposed with optical axises thereof according to each other, and the actuator includes a first actuator for three-dimensionally moving the first lens for correcting a focus error and a tracking error and a second actuator for inclining the second lens independently against the track crosswise direction of the optical recording medium and against a direction crossing the crosswise direction for correcting a tilt.
Accordingly, the second actuator can be driven independently for correcting a radial tilt and a tangential tilt. Also, the second lens is used for correcting a tilt alone, and hence, the second lens can be controlled to be inclined through small movement of the actuator.
In the apparatus for correcting aberration, the objective lens is provided, on a surface thereof opposing the optical recording medium, a contact preventing disk for retaining a distance to the optical recording medium at a predetermined or larger distance.
Accordingly, when the objective lens is brought too close to the optical recording medium, flowing air pressure affects the contact preventing disk, so as to prevent a distance between the objective lens and the medium from becoming smaller than the predetermined distance. Thus, a contact therebetween is avoided.
In the apparatus for correcting aberration, at least one of the photodetectors includes, on a light receiving surface thereof, a first light receiving area formed on one side along the track crosswise direction in the reflected light and a second light receiving area formed on the other side, and each of the first and second light receiving areas is symmetrically divided into at least four areas along the track crosswise direction in the reflected light.
When a defocus level is increased, the number of brightness peaks in the interference areas is increased. Therefore, the range of the defocus level applicable to the focus error correction can be enlarged by further dividing the first and second light receiving areas along the track crosswise direction.
In the apparatus for correcting aberration, the photodetector includes, on a light receiving surface thereof, a first light receiving area formed on one side along the track crosswise direction in the reflected light and a second light receiving area formed on the other side, and each of the first and second light receiving areas is linearly divided substantially parallel to the track crosswise direction and a direction crossing the track crosswise direction respectively.
Accordingly, since the first and second light receiving areas each in the substantially elliptical shape are linearly divided, the light receiving surface can be easily divided. Also, when the first and second light receiving areas are further divided parallel to the track crosswise direction in the reflected light and the direction crossing the track crosswise direction, one photodetector can be applied to the detection of all the causes such as a focus error, a tracking error, substrate thickness variation, a radial tilt and a tangential tilt.
In the apparatus for correcting aberration, the photodetector has a first photodetector part including a first light receiving area disposed on one side of a light receiving surface along the track crosswise direction in the reflected light and a second photodetector part including a second light receiving area disposed on the other side, and the first and second photodetector parts are movable toward and away from each other along the track crosswise direction.
Accordingly, although a distance between the interference areas of the reflected light is varied depending upon the track pitch of an optical recording medium, since the photodetector parts respectively including the first and second light receiving areas are thus separated, the distance between the first and second light receiving areas can be adjusted in accordance with the optical recording medium by moving the photodetector parts.